Polymerisable liquid crystal (LC) materials are commonly used for the preparation of optical films in liquid crystal displays. These materials usually contain a certain amount of compounds with two or more polymerisable groups (di- or multi-functional), which are crosslinked to give a hard film.
However, during polymerisation certain polymerisable materials, like for example acrylates, suffer polymer shrinkage [see R. A. M. Hikmet, B. H. Zwerver and D. J. Broer Polymer (1992), 33, 89]. This shrinkage causes a lot of strain in the polymerised film and reduces the adhesion between the film and the substrate.
One technique reported in prior art to overcome this problem concentrates on modifying the substrate to improve its adhesion to the polymerised film. For example, the substrate can be subjected to special treatment, for example flame treatment as disclosed in U.S. Pat. No. 2,795,820 or GB 0 788 365, corona treatment as reported in DE 1 128 644, or plasma treatment as reported in R. L. Bersin Adhesives Age (1972) 15, 37.
Alternatively, a separate adhesion or coupling layer (typically a solution of organo-silane materials) can be coated onto the substrate to help increase adhesion of a polymer film to a substrate, like e.g. the commercially available Addid 900 ® (from Wacker GmbH, Burghausen, Germany), an amino-functional trimethoxy silane.
U.S. Pat. No. 5,631,051 discloses a method of preparing an optical compensation sheet on a transparent substrate of triacetyl cellulose (TAC), by first providing an adhesion layer of gelatine on the TAC film. Then an aligning layer is formed by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerisable groups, onto the gelatine layer, evaporating the solvent and rubbing the surface of the polymerised PVA layer unidirectionally, Finally an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerised.
U.S. Pat. No. 5,747,121 discloses a method of preparing an optical compensation sheet by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerisable groups, onto a transparent substrate, evaporating the solvent and rubbing the surface of the PVA layer unidirectionally. Then an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerised. Afterwards the film is subjected to heat treatment whereby the PVA layer and the discotic LC layer are reported to be chemically bonded to each other via free, crosslinkable groups.
However, the use of separate adhesion or aligning layers comprising isotropic materials like gelatine or PVA can negatively influence the performance of the liquid crystal film when used e.g. as an optical film.
To overcome this problem an adhesion promoter can be directly added to the polymerisable LC material formulation. For example, WO 2006/062352 A1 discloses a homeotropic aligned polymer film without using a separate adhesion or aligning layer. The polymer film is obtained by coating a polymerisable LC mixture solution including a predetermined surfactant on a plastic substrate whose surface is hydrophilic-treated, drying, and UV irradiating the mixture solution.
However, all of the above-described methods have the distinct disadvantage in that they involve extra processing steps, like a surface treatment step.
Therefore, there is still the need for an alternative polymerisable liquid crystal (LC) material for the preparation of a homeotropic aligned polymer film, which does not exhibit the drawbacks of prior art materials or if so, do only exhibit them to a less extent.
Advantageously, the polymerisable LC material for the preparation of a homeotropic aligned polymer film, should                show favourable adhesion to a plastic substrate, such as TAC, COP, or colour filters etc.        do not require the use of separate adhesion layer,        do not require the use of a separate alignment layer,        exhibit an improved uniform homeotropically alignment,        be highly transparent to VIS-light, and        show a favourable high temperature stability, especially in view of in-cell applications.        
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
Surprisingly, the inventors of the present invention have found that the above aims can be achieved and the mentioned drawbacks of prior art polymerisable LC materials can be overcome by using a polymerisable LC material comprising a low amount of an adhesion promoter. The adhesion promoter should improve adhesion of the polymerised LC film to the substrate, and at the same time should not, or only to a minor extent, negatively affect the liquid crystal phase of the material, such as the uniform orientation, or the optical properties of the film, such as the transmission.